Cathode for electron discharge devices



Patented June 11, 1940 UNITED STATES PATENT OFFICE CATHODE FOR ELECTRON DISCHARGE DEVICES aware No Drawing. Application April 29, 1939, Serial No. 270,909

15 Claims.

My invention relates to electron discharge devices, more particularly to improvements in thermionic electron emitting cathodes for such devices.

In conventional electron discharge devices with the filament type of thermionic cathode commercial use has been made of pure tungsten filaments, activated thoriated tungsten filaments, and oxide coated nickel filaments. The pure tungsten filament cathode is a very stable emitter and is efiicient but must be operated at very high temperatures to obtain ample emission. The activated thoriated tungsten cathode, such as described in U. S. patent to Langmuir 1,244,216, may be operated at lower temperatures than a pure tungsten filament and is more efficient, but is easily poisoned by oxygen and other gases which adversely affect the emission, is not very stable when operated at high voltages, and requires special processing and seasoning. It has also been found that when operated at ultra-high frequencies, for example above 15 or 16 megacycles, that emission becomes erratic and that there is sometimes a total loss of emission. In addition, the activated thoriated filament is apt to be brittle, and if operated at too high a temperature quickly deactivates and loses its ability to emit a sufficient number of electrons to be useful. The oxide coated nickel cathode operates at a much lower temperature than either kind of tungsten cathode and is efficient, but cannot be satisfactorily operated in tubes in which very high plate voltages, such as 1000 volts, are used, and requires considerable processing and seasoning.

Thoriated molybdenum has also been thoroughly investigated as a filament cathode emitter, but has proved to be rather unstable, quite sensitive to poisoning by gas within the tube, and must be operated at fairly high temperatures, between 1400" and 1600 C. to provide satisfactory emission. For these and other reasons thoriated molybdenum has not come into commercial use as a cathode filament material.

In the conventional high vaccum or gas tube operating at high voltages, only filamentary type cathodes have heretofore been used because the oxide coatings of the indirectly heated cathodes quickly disintegrate at high voltages, and without these coatings no material was available which had sufficient emission unless raised to very high temperatures which cannot be practically generated with the conventional indirectly heated cathode construction.

The object of my invention is to provide a ductile thermionic cathode which is stable and efficient at desirable operating temperatures. A further object is to provide an eflicient thermionic cathode which can be satisfactorily used at high frequencies and high voltages and requires little seasoning or processing. Another object of my invention is to provide an indirectly heated cathode for high voltage gas or vacuum tubes.

In accordance with my invention I use a refractory metal having a high melting point, preferably 2400" C. or higher combined with thoria or its equivalents and cathodically treated in an electrolytic bath. Of these refractory metals I prefer to use tungsten and molybdenum. The thoria may be incorporated in the tungsten or molybdenum metal to form a base, and then treated according to my invention, or the pure tungsten or molybdenum may be specially treated according to my invention and the base then coated with thoria.

The thoriated molybdenum or thoriated tungsten base may be prepared as a ductile metal in acordance with U. S. Patent 1,082,933 to W. D. Coolidge, or by squirting the finely divided material admixed with a binder. For example, if molybdenum is used I may add powdered nitrate of thoria to the powdered oxide of molybdenum before the reduction of the oxide of molybdenum or I may add thoria to the oxide of molybdenum after reduction but before consolidation of the metal powder by sintering or mechanical working to the solid state as described in the said Coolidge patent. The amount of thoria may vary from about or 1% by weight of the metal to about of the metal. Tungsten containing more than 2% or 3% of thoria is extremely brittle and hard to work, but in general I have found it desirable for best results to add as much thoria as may be tolerated in the refrac- 40 tory metal without interfering with the subsequent swaging and drawing of the metal. This has been found in practice to be up to 1 for tungsten and up to 3% for molybdenum. A filament of this thoriated metal may be used. In general the larger the percentages of thoria the better the emitter. While thoria is preferred equivalents of thoria, for example one of the group of metals consisting of zirconium, uranium, cerium, titanium, vanadium, yttrium and lanthanum may be used with tungsten and molybdenum to form a base and the base then cathodically treated in an electrolyte, preferably dilute sulphuric acid which may be /g% solution of H2804. Other acids such as chromic acid (HzCl'Os), acetic acide. H(C:Ha0a) and hydrochloric acid (HCHl) also provide suitable baths. ing the thoriated metal base may vary between wide limits, for example, from a few hundredths of an ampere to over 1 ampere per square centimeter, for best results a current density of about 1 ampere per square centimeter for a period of one minute sems to produce the most satisfactory results. The resulting thoriated wire may be used for a thermionic electron emitting cathode without further treatment, but for best results in the case of the thoriated tungsted base I perfer to fiash the electrolyzed filament in vacuum at over voltage; for example, to heat the filament to a temperature of approximately from 1500 to 2000 K. for short periods up to ten minutes, and thereby facilitate the activation of the filament. It is also desirable, though not necesary, to insure stability of emission to season the filament whether of tungsten or molybdenum by operating the electron discharge device containing the filament with normal voltages applied for approximately fifteen minutes in an oscillating circuit.

A cathode made in accordance with my invention has a normal operating temperature several hundred degrees K. below the normal operating temperature of the conventional activated thoriated tungsten cathode, and in the conventional type of tube operated at the customary plate voltage the same plate current can be obtained from my improved cathode at its normal operating temperature with only about one-half to two-thirds the energy per unit area that must be used to obtain the same plate current from the conventional activated thoriated tungsten cathode at its normal operating temperature.

While I do not wish to be limited to any particular theory it is believed that the following explains the superior qualities of a cathode filament made according to my invention.

When the thoriated tungsten or thoriated molybdenum filament is cathodically electrolyzed, the tremendous electrostatic field acting at the electrode during electrolysis causes hydrogen ions to be sorbed into the crystal lattice of the When subsequently the hydrogen is sudmetal. denly released by heating, the crystal structure of the surface of the filament is shattered, causing the metal surface to become semi-liquid for an instant of time. During this instant of semiliquid state the surface tension becomes effective to cause the metal surface to pass to the amorphous state and subsequently freeze in this state so that it does not recrystallize. This results in a greater surface energy of the filament and makes it capable of adsorbing more thorium with greater energy.

In a modification of my invention a pure molybdenum or tungsten filament which has been previously electrolyzed as described above is brushed with powdered thoria suspended in an acetone solution to provide a coating which results in an efiicient electron emitting cathode. This coating when applied to a cathode filament of either pure tungsten or molybdenum. which has been treated according to my invention will provide a very efficient electron emitting cathode. No binder is necessary and if the cathode is handled carefully the coating will not come off while the filament is handled during assembly in a tube in which it is to be used. When the cathode is heated during operation of the tube the coating is sintered on the cathode filament.

While the current density for clectrolyz- Although the thickness of the coating is not critical it should not be so thick as to materially increase the surface area of the cathode and thus lower the temperature of the cathode to such an extent as to affect emission when the cathode is operated at what are considered normal currents and voltages. To insure stability of emission it is desirable, though not necessary, to operate the tube with normal voltages for about fifteen minutes in an oscillating circuit. Conventional methods of ageing can also be used for this purpose, although I prefer to use the oscillating circuit.

A cathode made in accordance with my invention is a very efficient emitter and it will operate at a comparatively low temperature.

Thoriated tungsten filaments treated according to my invention indicate emission currents of 6.0 amperes per cm. when operated at watts per cm. in comparison with 2.89 amperes/cm.

with untreated thoriated tungsten, burning at the same temperatures, Pure tungsten electrolyzed and coated with pure thoria also indicates emission currents of substantially the same magnitude. When such a filament is operated at watts per cm. which gives a temperature well below 2000 K. a value of 7.0 amperes per cm. is indicated. Making a comparison in another manner at 20 watts per cm. the emission etliciency of thoriated tungsten made according to my invention is 150 ma. per watt. For the usual carburized thoriated tungsten, the emission is between 110 and 115 ma. per watt at 30 watts per cm. input. From these data it is seen that the electrolyzed thoriated tungsten is a more eflistable emitter and is not easily poisoned by gas.

The cathode readily recovers its emitting characteristics even if accidentally operated temporarily at too high temperatures. Low voltage ionization will not destroy the tube due to ion bombardment, and the cathode will recover its emission if temporarily impaired by being subjected to high voltages.

While I have indicated the preferred embodiment of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure uesd and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

What I claim as new is:

l. A thermionic cathode for use in an electron discharge device and comprising a metal of high melting point cathodically electrolyzed in a dilute acid, in combination with one of the group of metals consisting of thorium, zirconium, uranium, cerium, titanium, vanadium, yttrium and chromic acid, sulphuric acid, acetic acid and hydrochloric acid, said cathode also including thorium.

3. A thermionic cathode for use as an electron emitter in an electron discharge device comprising a refractory metal body consisting predominantly of one of the metals of tungsten or molybdenum cathodically electrolyzed in a dilute solution of acid and containing thorium.

4. A thermionic cathode for use as an electron emitter in an electron discharge device comprising a refractory metal body consisting predominantly of one of the metals tungsten or molybdenum having an amorphous surface produced by cathodically electrolyzing the emitter in a dilute solution of acid, and containing thorium.

5. A thermionic cathode for use in an electron discharge device and comprising tungsten containing thorium and cathodically electrolyzed in a dilute solution of one of the following group of acids, chromic acid, sulphuric acid, acetic acid and hydrochloric acid.

6. A thermionic cathode for use in an electron discharge device and comprising molybdenum containing thorium and cathodically electrolyzed in a dilute solution of one of the following group of acids, chromic acid, sulphuric acid, acetic acid and hydrochloric acid.

7. A thermionic cathod for use in an electron discharge device comprising one of the group of metals containing molybdenum and tungsten cathodically electrolyzed in a dilute solution of acid and coated with thoria.

8. A thermionic cathode for use in electron discharge devices and comprising tungsten cathodically electrolyzed in a dilute solution of one of the following group of acids, chromic acid,

prising cathodically electrolyzing a metal of high melting point in a dilute solution of acid and coating it with thoria.

11. The method of forming a thermionic cathode in an electron discharge device comprising cathodically electrolyzing a filament of thoriated metal in a dilute solution of acid.

12. A method of forming a thermionic cathode for an electron discharge device comprising mixing one of the group of metals tungsten and molybdenum with thoria and forming a cathode body, immersing said body in a dilute solution of acid and passing a current through said solution and said filament to electrolyze said filament.

13. The method of providing a thermionic cathode for an electron discharge device comprising mixing tungsten with thoria and forming a filament, immersing said filament in a dilute solution of one of the following group of acids, chromic acid, sulphuric acid, acetic acid and hydrochloric acid, and passing a current through said solution and said immersed filament to cathodically electrolyze said filament, and heating said filament momentarily to between 1500 and 2000 K.

14. The method of forming a thermionic cathode for an electron discharge device comprising mixing one of the group of metals tungsten and molybdenum with one of the group of metals thorium, zirconium, uranium, cerium, titanium, vanadium, yttrium and lanthanum and forming a cathode, immersing said cathode in a dilute solution of one of the following group of acids, chromic acid, sulphuric acid, acetic acid and hydrochloric acid, and passing a current through said dilute solution of acid and said cathode to cathodically electrolyze said cathode.

15. The method of forming a thermionic cathode comprising forming a cathode body of one of the group of metals comprising tungsten and molybdenum, and immersing said cathode body in a weak solution of acid, passing a current through said acid and said cathode body to cathodically electrolyze said cathode body, and coating said cathode body with thoria.

VICTOR 0. ALLEN. 

